Adjustable inductor



Jan. 4, 1949. G N 2,458,071

ADJUSTABLE INDUCTOR Filed Aug. 1, 1944 A ENT Patented Jan. 4, 1949 VIUNITED, STATES PATENT oarce ADJUSTABLE INDUCTOR James F. Gordon, Towson, MIL, assignor to Bendix Aviation Corporation, South Bend, Ind., a corporation of Delaware I Application August 1, 1944, Serial No. 541,650

6 Claims. 1

This invention relates to adjustable inductors and more particularly to inductors of this type in which the desired variation in characteristic is secured by alteration of mechanical dimensions of the inductor structure.

To secure the performance required in present day radio communication apparatus, it is necessary that the various circuits be accurately tuned with respect to the incoming signal frequency, and, in superheterodyne receivers, it is further necessary that the resonant frequency of the oscillator circuits maintain a special rerelationship to the resonance frequency of the signal responsive circuits, the two frequencies usually differing by an amount equal to the intermediate frequency of the receiver. To this end; it is customary to employ in the oscillator circuit a coil having inductance different from the coils present in the signal frequency circuit, since the commonly available gang capacitors have identical individual sections. The necessary difference in tuning ratio is made up by a padding. capacitor connected to the capacitor section controlling the oscillator circuit.

In the alignment of such'recei'veraitis desirable to control the minimumoscillator frequency as well as the tuning ratio of the circuit, and for this reason somemeans is usually provided for varying the inductance of the oscillator coil. At frequencies below two megacycles per second an adjustable iron core is frequently employed, whose uum tube amplifiers, and reduction in the operating Q of the circuit due to losses introduced by inductance controlling means greatly aggravates these dlfllculties.

To escape the disadvantages occurring from the use of conducting or high permeability adjustable coil cores, employment of air core coils has been i made. For the performance of the alignment or position within the coil is adjusted to provide the Y desired inductance. At higher frequencies. up to thirty megacycles per second, it becomes more difficult to provide ironwhose magnetic properties are satisfactory and there are, therefore, employed devices relying on magnetic fields produced by eddy currents to adjust the coil inductors. A typical example is the use of a brass plunger movable along the axis of the coil. Insertion of the plunger in the coil reduces the inductance thereof because of the eddycurrent reaction on the magnetic field. In this case, it is evident that power losses are reflected into the coil.

When working with signals of the'orderof 100 megacycles per second, it becomes quite difficult to produce iron having high enough permeability to permit adequate control of the inductance which, at the same time, has low losses, and the use of the eddy current method in itself relies upon the existence of losses for the control of inductance. Circuit impedances, as controlled by stray capacitors at 100 megacycles per second, are unfavorable to the efllcient operation of vactracking operation, the operator mechanically deforms the coil, changing its efl'ective length or turn spacing to control the inductance. The residual elasticity of the coil inductors makes it impossible to adjust directly to the desired value because of its tendency to return to the original shape. To overcome this, the operator is forced to distort the coil beyond the final desired configuration and trust that he has estimated the elasticity correctly enough to result in the required ultimate adjustment. This adjustment by successive approximation is time consuming and requires a skilled operator.

One of the principal objects of this invention is to provide a new and novel conveniently adjustable inductor having losses substantially no greater than that of the coil structureitself.

of graduated pitch formed therein, and placing the wires forming the inductance coil in said rooves.

Other objects and advantages of the invention will in part be disclosed and in part be obvious when the following specification is read in conjunction with the drawings in which Figure 1 is a view in perspective of the completed coil assembly.

Figure 2 is a plan view of the coil assembly. Figure 3 is a view in elevation of the core showing the wire position at one extreme of the ad-- justment, and

Figure 4 is a view in elevation of the core showing the wire position at the other extreme of ad- Justment.

Referring now to Figure 1, there is shown a core It provided with a helical spiral I! extending thereabout and having a variable lead or of apertures 22 and the slot 20 to the core in is most clearly seen in Figure 2.

To adjust the inductance of the coil formed by conductor I4, the blade of a screw driver is inserted in the slot 20 or a thin rod in one of the apertures 22 and the core Ill rotated about its longitudinal axis by movement of said tool. This displaces the wire in the groove I2 toward one end or the other of the graded pitch groove i2, lengthenin the coil or shortening same depending on whether rotation causes displacement of the coil towards the upper or lower end respectively.

This effect is clearly shown in Figures 3 and 4. Clockwise rotation of the core in drives the coil I4 toward the slotted end of the form, thereby extending the effective length of the coil and decreasing the inductance thereof. At the extreme of this adjustment, the conductor ll occupies the position in the groove. 12 indicated in Figure 3. Counter-clockwise rotation of the core I0 moves the coil in the opposite direction in the grooves i2, the extreme of this adjustment appearing in Figure 4 where the coil formed by conductor H has been displaced to the low end of the core; the one of minimum pitch, thereby adjusting the coil to its maximum inductance value. I

As seen in Figures 3 and 4, it is desirable that the root diameter of the grooves be somewhat less than the inner diameter of the coil formed by conductor II to prevent the coil from wrapping on the roots of the grooves and freezing the adjustment. In furtherance of this end, it is also desirable that the core Ill be of a material having a.low coemcient of friction with respect to the conductor ll. The core in may be constituted of any of the many plastic combinations having suitable characteristics for the intended working frequency range, or may be of ceramic material in which case the afllxing of a smooth vitreous glaze may assist in producing the desired low coefflcient of friction.

It will be obvious that many changes and modifications may be made in the invention without departing from the spirit thereof as expressed in the foregoing description and in the appended claims.

I claim:

1. In an inductive device, a ceramic core having a spiral groove of graduated pitch making a predetermined number of convolutions about said core and surfaced with a friction reducing glaze, and a helically formed conductor slideably engaging said groove.

2. In an inductive device, the combination of a helically formed conductor having a fixed number of turns, and a core rotatably supported within said conductor helix provided with turn spacing means introducing varying spacing, said turn spacing means engaging a plurality of turns of said conductor helix along substantially their entire length and movable axially of said conductor helix upon relative rotation between said core and said conductor helix. 1

3. In an inductive device, the combination of a helically formed conductor having a fixed num-- ber of turns, a member movable along the axis of said conductor helix upon the rotation of said member with respect to said conductor helix, said member being provided with turn spacing means slideably engaging at least one turn of said con ductor helix and spacing said engaged turns by varying distances in response to said rotation.

4. In an inductive device, the combination of a helically formed conductor, and a member within said conductor helix having variable pitch turn spacing means engaging said helix and axially movable therein, whereby the overall length of said conductor helix may be varied.

5. In an inductive device, the combination of a helically formed conductor having at least one turn, and a core member provided with spiral grooving of varying pitch rotatably supported within said helix with said spiral grooving in sliding engagement with the turns of said conductor helix, said core member moving axially within said conductor helix upon relative rotation between said helix and said core member.

6. In an inductive device, the combination of a helically formed conductor having a fixed number of turns, and a member rotatable within said conductor helix provided with means engaging the turns of said helix and varying the overall length of said conductor helix upon rotation of said member.

JAMES F. GORDON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,493,713 Tykociner May 13, 1924 1,634,541 Conrad July 12, 1927 1,860,176 De Tar May 24, 1932 2,037,061 Bliss Apr. 14, 1936 2,163,644 Ware June 27, 1939 2,635,541 Ware July 12, 1927 2,294,881 Alford Sept. 8, 1942 

